wrapping nodejs stream in JSON object

I have a readable stream, something like this:

const algorithm = 'aes-256-ctr';
stream = file.stream
    .pipe(crypto.createCipher(algorithm, encryptionKey))
    .pipe(outStream);

Encryption works as expected on the entire file. I need to wrap the result of crypto into some sort of json, so the output stream to receive something like this:

{
    "content": "/* MY STREAM CONTENT */"
}

how do I do that?

Additionally, I need to read the file stored on disk and unwrap it from json if the encryption key matches.

Thanks in advance

As of node v13, you can use generators within pipeline and build your object as a string:

// const { pipeline } = require('stream/promises'); // <- node >= 16
const Util = require('util');
const pipeline = Util.promisify(Stream.pipeline);

const algorithm = 'aes-256-ctr';
const Crypto = require('crypto');

async function run() {
  await pipeline(
    file.stream, // <- your file read stream
    Crypto.createCipher(algorithm, encryptionKey),
    chunksToJson,
    outStream
  );
}

async function* chunksToJson(chunksAsync) {
  yield '{"content": "';
  for await (const chunk of chunksAsync) {
    yield Buffer.isBuffer(chunk) ? chunk.toString('utf8') : JSON.stringify(chunk);
  }
  yield '"}';
}

Assuming a more complex case where a large amount of data is being streamed (which is usually the case when using streams), you may be tempted to do something like the following. This is not a good practice since all of the content will build up in memory before yielding, defeating the purpose of streaming.

async function* chunksToJson(chunksAsync) {
  const json = { content: [] };
  for await (const chunk of chunksAsync) {
    json.content.push(Buffer.isBuffer(chunk) ? chunk.toString('utf8') : JSON.stringify(chunk));
  }
  yield JSON.stringify(json);
}